This invention relates generally to voltage boost circuits and, more particularly, to a monolithically integrable circuit for sensing a high voltage, which circuit is manufactured using a low voltage process.
The voltage booster of a bubble memory operational driver requires circuitry for sensing when the voltage on an inductor decreases to a value approximately equal to the supply voltage (e.g. 5 volts). This is necessary to determine the earliest time at which a pull-down output transistor may be turned on which in turn permits current to flow through the inductor. At some time thereafter, current flow through the pull-down transistor is terminated (i.e. the pull-down transistor is turned off). As a result, a voltage is induced across the inductor which results in charge being stored in the capacitor. When the voltage on the inductor decreases as described earlier, the pull-down transistor is turned on, and the process is repeated until a predetermined high voltage is built up across the capacitor.
In a prior art approach, the voltage on the inductor is sensed by a resistor coupled to the emitter of a first transistor, the base of which is coupled to a source of supply voltage (typically five volts). The collector of this transistor supplies base drive to a second transistor having an emitter coupled to ground which, when turned on, diverts base current away from the pull-down transistor turning it off and terminating current flow through the pull down transistor. Thus, when the terminal of the resistor connected to the inductor exceeds the supply voltage by a predetermined amount, the first and second transistors are turned on maintaining the pull-down transistor in an off condition.
Unfortunately, this prior art approach has certain disadvantages. First, the regulated boost voltage which, as indicated earlier, may be as high as 50 volts results in excessive current loss in the resistor (e.g. approximately 8.3 milliamperes if the resistor is 6 K-ohms). If the resistor were increased to approximately 250 K-ohms, the current loss would be reduced to approximately 200 microamperes; however, at a resistivity of 5 K-ohms per square, an excessive amount of die space would be required (i.e. a 50 square resistor). The problem is further complicated when a low voltage semiconductor manufacturing process is employed; i.e. BV.sub.ceo equals approximately 18 volts.